1. Field of the Invention
The invention relates to measurement of physical properties of surfaces, in particular semiconductor surfaces, using non-destructive, non-intrusive analytic techniques. In particular,the non-destructive techniques involve the use of second harmonic generation by reflecting femtosecond laser pulses from a surface to be studied.
2. Description of the Related Art
Current trends in the silicon microelectronics industry are driving metal-oxide semiconductor (MOS) devices to deep sub-micron dimensions, and corresponding gate oxide layers are being driven to sub-50 Angstrom (.ANG.) thicknesses, thereby tightening the requirements for the control of silicon/silicon dioxide [Si(100)/SiO.sub.2 ] interfacial contamination and microroughness during device processing. FIGS. 1a and 1b illustrate representative dimensions used in current semiconductor devices and the dimensions expected in the semiconductor devices of the next decade, respectively.
Certain cleaning chemistries that are used on semiconductor devices can roughen the silicon surface. This roughening, even at the .ANG. level, can result in poor reliability for MOS devices. At present, few analytical techniques exist for measuring Angstrom-scale interface roughness in situ, without removing the oxide layer. Optical techniques are attractive for this purpose since they are non-destructive, non-intrusive, and can measure roughness at the Si(100)/SiO.sub.2 boundary through the thick transparent SiO.sub.2 layer. However, linear optical techniques, such as light scattering, as described by J. Bennett and L. Mattson, in Introduction to Surface Roughness and Scattering (Optical Society of America, Washington, D.C., 1989), probe the entire optical absorption depth and thus possess limited sensitivity to a specific buried interface of interest.
Optical surface second harmonic generation (SHG), on the other hand, can be an interface-specific probe for centrosymmetric semiconductors, since in the electric dipole approximation of such devices, even-order nonlinear susceptibilities are nonvanishing only at interfaces, where bulk inversion symmetry is broken. For example, in Si(100)/SiO.sub.2 structures, the electric dipole susceptibility of only a few atomic monolayers at the interface generates p-polarized (or parallel-polarized) second harmonic signals. Simultaneously, the bulk quadrupole susceptibility throughout an absorption depth of the material generates much weaker s-polarized (or perpendicular-polarized) second harmonic signals.
Previous studies of optical surface second harmonic generation of centrosymmetric semiconductors have used highly amplified laser pulses of nanosecond (ns) or picosecond (ps) duration in order to achieve observable second harmonic yields. These methods have been described by Y. Shen, in Principles of Nonlinear Optics (John Wiley and Sons, 1984). These approaches suffer in their measurement capabilities due to low repetition rate (around 10 Hz), limited signal-to-noise ratio, long acquisition times, as well as significant heating of the sample being measured, whereby the heating may even cause damage to the semiconductor device being measured.
In light of the above problems with the current techniques, it is desirable to come up with a technique for measuring the roughness of internal layers of semiconductor devices using non-destructive and non-intrusive methods, and also doing so with a better measurement capability than what is currently available.